Acer battery not fully charging

It's nothing you can fix.

Either the battery has issues or it's the charger. Probably it's the battery. All batteries drop slowly with time.

You probably have one of the issues under "other problems perceived as memory effect":
http://en.wikipedia.org/wiki/Memory_effect

I have the same problem. Computer only five months old when battery began only charging up to 82% then 66% and now on 53%

There have been battery recalls for laptops. Investigate this with your laptop company.

The secret with laptop batteries (in fact - any rechargeable battery in any appliance that offers mains or battery power) is to only use the battery when you are not near a convenient AC power outlet.

The worst thing you can do is to constantly run the laptop plugged into the mains with the battery fully charged. This will kill the battery in no time - a lot faster than repeated charge and discharge cycles, which will also lead to it's demise but at a far slower pace (all recharge batteries have a finite number of charge and discharge cycles before they WILL say cheers!).

So the rules are:

If away from AC power use the battery.
When back at your desk (or wherever) plug back into mains until the battery is fully recharged AND THEN DISCONNECT THE BATTERY FROM THE LAPTOP! Your laptop will now run on mains only and your battery will not suffer the damage caused by having a charge constantly rammed down it's throat.